This invention relates to the use of borate-polyol complexes in ophthalmic compositions. In particular, these complexes are useful as buffers and/or antimicrobial agents in aqueous ophthalmic compositions, including those ophthalmic compositions containing polyvinyl alcohol.
Ophthalmic compositions are generally formulated to have a pH between about 4.0 and 8.0. To achieve a pH in this range and to maintain the pH for optimal stability during the shelf life of the composition, a buffer is often included. Borate is the buffer of choice for use in ophthalmic compositions, since it has some inherent antimicrobial activity and often enhances the activity of antimicrobials; however, when polyvinyl alcohol (PVA) is also an ingredient in the composition, borate and PVA form a water-insoluble complex which precipitates out of solution and acts as an irritant in the eye. This incompatibility of borate and PVA in contact lens solutions is well-known, and has been discussed, for example, in an article by P. L. Rakow in Contact Lens Forum, (June 1988), pages 41-46. Moreover, borate buffer cannot be effectively used below pH 7.0 due to its low buffering capacity to lower pH.
Since borate is incompatible with PVA, ophthalmic compositions containing PVA are generally buffered with acetate, phosphate or other buffers. There are disadvantages to using these alternative buffers: for example, acetate is a weak buffer (pK.sub.a of about 4.5), so a relatively large amount is needed; on the other hand, phosphate is a good buffer but, when used in concentrations generally found in ophthalmic formulations, it reduces the antimicrobial activity of preservatives.
It is well known that small organic compounds, such as benzalkonium chloride (BAC), chlorhexidine, thimerosal have excellent antimicrobial activity; however, it is now known that these small organic antimicrobials are often toxic to the sensitive tissues of the eye and can accumulate in contact lenses, particularly soft, hydrophilic contact lenses. More recently, polymeric antimicrobials such as Polyquad.RTM. (polyquaternium-1) and Dymed.RTM. (polyhexamethylene biguanide) have been used in contact lens care products as disinfectants and preservatives. While these polymeric antimicrobials exhibit a broad spectrum of antimicrobial activity, they generally have relatively weak antifungal activity, especially against Aspergillus niger and Aspergillus fumigatus.
A need therefore exists for ophthalmic compositions which have an optimal pH for stability and efficacy, but whose antimicrobial efficacy is not compromised.